With recent developments in semiconductor technology, a semiconductor device has been practically realized which has a great number of functional elements formed on a single semiconductor substrate. For example, there has been realized a semiconductor device having within its semiconductor substrate a CPU (Central Processing Unit) for performing logical operations, data control and the like, a ROM (Read Only Memory) for storing fixed data such as the operating system (OS) and the like, a RAM (Random Access Memory) for storing temporary data, and other elements. Such a semiconductor device includes almost all functional elements necessary for a computer in one chip. A very compact device with great information processing capability is thus realized and has wide application such as in IC cards.
It has been desired to have such a semiconductor device with more sophisticated functions as its memory capacity increases and its logical circuits become large scaled circuits. The reason for this is that devices which can process more information at higher speeds are becoming necessary. In order to meet such requirements, it is necessary to integrate elements as much as possible on a single semiconductor substrate. It therefore becomes necessary to make each element smaller. If this is not possible, the chip area will increase to pose a number of problems associated with manufacturing techniques, resulting in difficulty in mass-producing such devices at a low cost.
In order to make an element small, precise control of lithography should be unsured and measures should be taken to account for degradation of reliability to be caused by miniaturization which results in difficulties associated with manufacturing techniques.